Shaped charges are commonly used to perforate casing in an oil or gas well, and a plurality of such charges are generally run into the well bore in a tubular perforating gun at the end of a wireline or on tubing. The gun holds each charge in a desired outward-pointing orientation and at a particular vertical level. As each charge is detonated, the explosive jet penetrates the casing, the cement sheath surrounding the casing, and extends into the producing formation, ideally forming a tunnel therein to provide more surface area and an enlarged flow path for the oil or gas from the formation.
In recent years, it has been recognized that certain producing formations benefit from so-called "high density" perforating, which may be defined as making more than twelve (12) perforations per foot of well interval. To effect such high perforation densities, shaped charges have been used in clusters of two, three, four and even five charges at 180.degree., 120.degree., 90.degree. and 72.degree. circumferential intervals, respectively. The clusters of shaped charges are mounted with the detonating ends of the charges pointed on radial lines toward the center of the perforating gun housing, and in close proximity thereto, and the mouths of the charges facing outward. A detonating cord extends down the centerline of the perforating gun, and is contacted about its periphery by booster charges at the detonating ends of the shaped charges. When the detonating cord is ignited by the firing head of the perforating gun, it detonates and in turn sets off the booster chargcs in the shaped charges, which initiate the shaped charge explosions.
An example of a prior art high-density perforating system is disclosed in U.S. Pat. No. 4,140,188, issued on Feb. 20, 1979 to Roy R. Vann. While such a high density system may be advantageous, it suffers from a serious deficiency in that the detonating cord may not effect sufficient energy transfer to the booster charges of all the shaped charges in a cluster on the same radial plane. The aforesaid deficiency is inherent in the detonating cord of the prior art, due to its cross-sectional configuration, which is generally circular, so that the detonating cord detonation results in a cylindrically expanding energy wave, which experiences an energy density decrease between the cord and the booster charges, proportional to the square of the distance travelled by the energy wave. Moreover, the dense clustering of charges about a central detonating cord severely limits the standoff distance of each charge from the wall of the gun housing. As adequate standoff is critical for maximum penetration of the shaped charge jet, the use of a cylindrical prior art cord having sufficient explosive material therein can impair jet efficiency by reducing standoff.
Prior art detonating tapes, fuses, or cords having one or two flat sides are known, but such tapes, fuses, or cords are not suitable for detonating a shaped charge due to their fragility and lack of sufficient energy propagation. Polygonal detonating cords of irregular cross-section are also known, as are cords having combinations of arcuate and flat sides, but these prior art cords are configured to propagate energy in a single direction.